The invention relates to an optoelectronic device for acquisition of images, in particular of bar codes.
The present optoelectronic devices which are designed for reading of bar codes conventionally comprise a case which contains an electronic scanning sensor, optical means which comprise a diaphragm, and make it possible to form images on the sensor, and to define an optical plane together with the latter, and electro-luminescent diodes which are designed to light the bar codes, and to permit acquisition of the images on the sensor. Finally, these devices comprise processing electronics which permit piloting of the sensor, and processing of the signals obtained from the latter.
In the conventional optoelectronic devices, the diaphragm has a circular aperture with a small diameter, such as to prevent defocussing of the image, and/or to increase the depth of field of the device. However, as a result of the small diameter of this aperture, the light intensity, which is reflected by the bar code and reaches the sensor, is reduced, and in practice this fact makes it necessary to use light sources which have high light intensity, such as to compensate for the reduction of light intensity.
It should be noted that it has been envisaged to increase the diameter of the aperture of the diaphragm, in order to increase the irradiance on the sensor. However, this solution leads to reduction of the depth of field of the device, and thus to reduction of the efficiency of the latter.
In order to eliminate these disadvantages, one solution consists of producing an optoelectronic device, as described in patent application EP 61000, the diaphragm of which has an aperture with a longitudinal axis which is at right-angles relative to the axis of the bar codes, such as an aperture which has a rectangular, rhomboid or elliptical shape.
In practice this solution makes it possible to increase the sensitivity of the optoelectronic devices, in proportion with the ratio of flow received relative to flow reflected. By this means, the depth of field of these devices is increased, without effecting substantially the intensity received on the sensor, and the efficiency of these devices is thus increased.
However, as a result of the relatively large dimensions of the aperture of the diaphragm, a solution of this type makes it necessary to use optical means for formation of the image on the sensor, which are larger than those of the conventional optical means, and which are thus more costly and complex to produce than those of the conventional optical means.
Another solution consists of producing optoelectronic devices as described in international patent application WO-9620454, comprising two dioptres at right-angles relative to one another, which are disposed between the diaphragm and the optical sensor, and are designed to obtain, in the optical plane (XOZ) parallel to the optical plane, enlargement ml which is greater than the enlargement m2 in the plane (YOZ) perpendicular to the optical plane.
A solution of this type, which can also be associated with that which is described in patent EP-61000, has optical means with characteristics which lead to an increase, according to axes which are parallel to the bar codes, in the dimension of the lit surface of the said bar codes, the image of which is reflected on the sensor, and thus to an increase in the sensitivity of the optoelectronic device.
It should also be noted that since this increase in the sensitivity of the device is obtained simply from the design of the optical means, and not from the dimensions of the aperture of the diaphragm, a device of this type can be equipped with a conventional diaphragm with a circular aperture which has small dimensions, and thus with optical means with conventional dimensions, which are cheap and easy to produce.
However, as specified in the patent application in question on page 39, lines 23-24, with reference to FIG. 53, the optical means which make it possible to obtain this result are very problematic to produce, which has led the inventor to abandon this solution, and to propose a different solution.
The object of the present invention is to eliminate this disadvantage, and its main objective is to obtain enlargement m1 in planes (XOZ) parallel to the optical plane, which is greater than the enlargement m2 in a plane (YOZ) perpendicular to the said optical plane, by using optical means which are in common use and are easy to produce.
For this purpose, the invention relates to an optoelectronic device for acquisition of images, in particular of bar codes, comprising a case which is provided with a reading window, and contains an electronic scanning sensor, lighting means, and optical means which comprise a diaphragm, and are designed to ensure that images are formed on the sensor, and to obtain, in a plane (XOZ) parallel to the optical plane, enlargement m1 which is greater than the enlargement m2 in a plane (YOZ) perpendicular to the said optical plane, the sensor and the optical means defining an optical plane relative to which the reading window is centred, and the said sensor, reading window and optical means delimiting a useful optical reading field.
According to the invention, the optical means comprise:
first optical means, comprising a first, converging lens, consisting of a symmetrical lens which is revolved around the optical axis, the useful part of which is convex; and
second optical means, comprising a so-called cylindrical lens with a semi-cylindrical dioptre, which is designed not to create any deflection in the plane (XOZ) parallel to the optical plane, and to converge in the plane (YOZ) perpendicular to the said optical plane.
According to the invention, the enlargement m1/m2 is obtained by means of lenses with a conventional design, i.e.:
a first symmetrical lens which is revolved by means of a simple conventional turn; and
a second, cylindrical conventional lens, of a design which is commonly available.
The invention thus leads to the same advantages as those described on page 39 of patent WO-9620454, whilst leading to conventional production of the optical means, the cost price of which does not make the cost of the optoelectronic device disadvantageous.
However, it has been found that although use of optical means which consist of a symmetrical lens which is revolved, and a cylindrical lens, lead to perfect results when the optoelectronic device is perfectly aligned with the bar code (product which is passed in front of a fixed optoelectronic device, etc), the astigmatism which optical means of this type introduce increases the sensitivity to errors of rotation around the optical axis.
For this reason, and according to another characteristic of the invention, the optical means additionally comprise a correction lens, which is associated with the first optical means. It should be noted that the construction of this correction lens is easy, since it is neutral in the plane XOZ in which the bar code is used.
In fact, the solution preferably consists of using a single- or multiple-component symmetrical lens which is revolved, which is associated with a cylindrical lens which has a meniscal-type profile for opposite enlargement, which is designed to cancel out the optical power in the plane (YOZ). This therefore retains the optical quality which is commonly achieved for symmetrical revolving optics necessary in the planes (XOZ). It should also be noted that the assembly of the main lens/correction lens must be optimised as a whole, since the cylindrical lens introduces an astigmatism at the edges of the fields.
Thus, the correction lens, which advantageously consists of a simple flat/concave cylindrical lens, constitutes a correction component which makes it possible to solve the problems of rotation, whilst maintaining the advantage of leading to an increase in the sensitivity of the device.
In practice, experiments have thus made it possible to show that the bar code can be inclined by an angle greater than 10xc2x0, before the MTF (Modulation Transfer Function) is reduced significantly.
According to another characteristic of the invention, the optical means are designed so that the ratio of m1/m2 is such that 3 less than m1/m2 less than 5.
In fact, this ratio range constitutes a good compromise for the performance of the optoelectronic device, which leads to a substantial increase of the sensitivity of the latter relative to the present conventional devices, without however detracting from the functioning of the said device, if it is positioned in front of the object which is being rotated around the optical plane (XOZ).
According to another characteristic of the invention:
the ratio of the enlargement m1/m2 of the first and second optical means is such that m1/m2 is greater than 1; and
the first optical means are disposed in the vicinity of the diaphragm, between the said diaphragm and the second optical means.
In addition, the lens which is revolved is advantageously of the aspherical type, and has the shape of an ogive, such as to prevent geometric aberrations.
According to a further characteristic of the invention, the linear sensor and the diaphragm are disposed such that they are centred on axes at right angles relative to one another, a mirror which is inclined by an angle of 45xc2x0 relative to the said axes being positioned such as to reflect the images on the said sensor.
This arrangement has the advantage that it leads to a reduction in the size of the optical means, and thus to optimisation of the size of the case.
According to a further characteristic of the invention, the diaphragm has an aperture with a dimension in the direction Y which is greater than that in the direction X. This leads to an increase in the sensitivity of the device or the depth of field.